Box2D uses radians for angles. The body rotation is stored in radians and may grow unbounded. Consider normalizing the angle of your bodies if the magnitude of the angle becomes too large (use b2Body::SetAngle)

After getting confused for a while I realized this was old text from the Box2D manual (b2Body doesn't have a GetAngle method).

Here's my suggested change to the text:

Box2D uses radians for angles. The body rotation is stored in radians and may grow unbounded. Consider normalizing the angle of your bodies if the magnitude of the angle becomes too large (e.g. use b2Body.angle %= 2*pi)